1. Field of the Invention
The present invention relates to an electrostatic lens array that is used for a charged particle optical system.
2. Description of the Related Art
A multi-charged-particle beam drawing apparatus uses an electrostatic lens array for converging a charged particle beam (see Japanese Patent Application Laid-Open No. 2005-032480 and International Publication No. WO2011/043668). The multi-charged-particle beam drawing apparatus accelerates, shapes, and reduces charged particles emitted from a charged particle source, and irradiates a sample with the charged particle beam so as to draw a desired pattern on the sample. When shaping the charged particle beam, the electrostatic lens array is used for converging multiple charged particle beams separately.
In the invention described in Japanese Patent Application Laid-Open No. 2005-032480, the electrostatic lens array is a lens for an image before a reducing optical system. Therefore, an influence of a defocus amount is small, which is generated due to a thickness variation of electrodes constituting the electrostatic lens. In contrast, in the case of a multi-column type charged particle optical system including the electrostatic lens array for an objective lens that is a reducing optical system, the influence of a defocus amount which is generated due to a thickness variation of electrodes is large.
As another example using an electrostatic lens for an objective lens, in the case of an electron microscope using a single particle beam for example, even if a thickness variation of electrodes constituting the electrostatic lens occurs, the electrostatic lens that functions as an electrostatic lens at one time is a single aperture. Therefore, focus can be adjusted in the vicinity of the aperture, and there is no essential problem.
However, in the case of an apparatus which uses multiple particle beams, it is necessary that many particle beams are focused simultaneously by the electrostatic lens array having many apertures.
In addition, in the invention described in International Publication No. WO2011/043668, the electrostatic lens array used for the objective lens is fixed to a lens assembly mainly with an adhesive. Therefore, even after the adjustment of parallelism between the lens assembly and a wafer, the electrostatic lens array and the wafer are not always parallel to each other. Therefore, in the charged particle optical system using the electrostatic lens array for the objective lens, it is necessary to perform a difficult adjustment for minimizing a total focus deviation by evaluating an image formed by the multiple electrostatic lenses.
As described above, this focus adjustment takes long time in the conventional apparatus.